1. Field of the Invention
This invention is directed to overbased alkylated alkyl salicylates which overbased products are suitable for use in lubricating oil compositions used in internal combustion engines. In particular, these additives impart water tolerance, thermal stability, detergency, compatibility and good oxidation performance in such lubricating oil compositions.
2. State of the Art
Overbased alkylated salicylates are a class of lubricating oil detergents known to impart improved performance (detergency, water tolerance, thermal stability, antioxidancy) to lubricating oil compositions used for lubrication of internal combustion engines.
Overbased salicylates are prepared by overbasing the corresponding alkylated salicylic acids which, in turn, are typically prepared by first alkylating phenol to form alkylphenol followed by carboxylation via the Kolbe-Schmitt reaction to provide for alkylated salicylic acid. The alkyl group is typically a long chain alkyl group of greater than about 14 carbon atoms so as to impart oil solubility.
One problem encountered with this synthetic scheme when a substantially linear alkylation feed is employed is that not all of the long chain alkylphenol is readily carboxylated via the Kolbe-Schmitt reaction. Specifically, conventional alkylation of phenol with a substantially linear alkylation feed provides for approximately a 50:50 mixture of ortho-alkylphenol and para-alkylphenol. While the Kolbe-Schmitt reaction readily carboxylates the resulting long chain para-alkylphenol, the resulting long chain ortho-alkylphenol is less reactive and only about 70 percent of total amount of the alkylphenol derived from a substantially linear alkylation feed is typically converted to alkylated salicylic acid during this reaction.
One method of circumventing this problem is to alkylate an alkyl salicylate (e.g., methyl salicylate) and then subject the resulting alkylated alkyl salicylate to hydrolysis so as to provide for the alkylated salicylic acid. Methods of alkylating alkyl salicylates are disclosed in U.S. patent application Ser. No. 08/72,544, filed concurrently herewith and entitled "ALKYLATION OF ALKYL SALICYLATE USING A LONG CHAIN CARBON FEED" which application is incorporated herein by reference in its entirety.
This latter synthetic scheme involves formation of an alkyl salicylate, alkylation of the alkyl salicylate to form an alkylated alkyl salicylate, followed by hydrolysis to form the alkylated salicylic acid. Hydrolysis of the alkylated alkyl salicylate to form alkylated salicylic acid was perceived necessary to later formation of overbased products since the carboxyl (--COOH) group was deemed to be an essential component during overbasing. Specifically, overbased products are prepared by the addition of excess basic metal (i.e., greater than that required to neutralize all of the acidic species on the salicylic acid) optionally using carbon dioxide and it was considered that the carboxyl group played an essential role in the ability of the alkylated salicylic acid to incorporate excess basic metal and carbon dioxide presumably via chelation.
This invention is directed, in part, to the discovery that alkylated alkyl salicylates can be overbased without hydrolysis of the ester functionality to provide for overbased lubricating oil additives suitable for use in lubricating oil compositions. This discovery is particularly surprising in view of the fact that the overbased products of this invention lack carboxyl (--COOH) functionality but, rather, possess an ester (--COOR) functionality (R being alkyl of from 1 to 6 carbon atoms).